In a variety of situations, it is desirable to be able to jam a receiver from receiving an intended signal. To address this need, jammers have been developed that output waveforms which create a large amount of noise, making recognition of the intended signal difficult.
Jamming is usually achieved by transmitting a jamming signal at a same frequency or in a same frequency band as that used by the intended signal. The jamming signal may block a single frequency, identified as “spot jamming,” or may block a band of frequencies, identified as “barrage jamming.”
Although simple jammers have long existed, technological advances required the development of improved jamming equipment. Early jammers were often simple transmitters keyed on a specific frequency thereby producing a carrier which interfered with the normal carriers at targeted local receivers. However, such single carrier jammers have become ineffective and easily avoided using, for example, frequency hopping, spread spectrum and other technologies.
Current jammers are typically either barrage jammers broadcasting broadband noise or continuous wave (CW) signals targeted at specific known intended signals. Generally, barrage jammers tend to produce a low energy density in any given communications channel, for example a 25 kHz channel, when jamming a broad band of channels. By way of example, a 200 MHz barrage jammer transmitting 100 Watts generally will have 12 mWatts in any communications channel and this low power level per channel is likely to be ineffective as a jammer. These jammers also tend to jam wanted communications.
A common jammer technique is to capture an individual local transmitter signal for a short period of time, copy the captured signal as a regenerated signal and retransmit that regenerated signal a short period of time later. Such a “regenerative” jammer creates false radar targets that appear as real targets thereby confusing the radar local receivers. In U.S. Pat. No. 6,476,755, a jammer uses time-division multiplexing techniques that permit monitoring received RF local transmitter signals while, in a time-division multiplexing sense, concurrently transmitting RF signals to jam selected transmissions at local receivers. The time-division multiplexing alternately enables the jamming system receiver and transmitter with operation at a frequency higher than the Nyquist rate. The precision required in the receiver of such a jammer may be cost prohibitive, however.
As explained above, different receivers may use different modulation formats and over the air protocols. For example, some receivers recognize frequency modulated (FM) signals, while others recognize amplitude modulated (AM) signals, pulse modulated signals, or signals modulated by other modulation schemes. To create jammers that are capable of outputting a signal to jam a given receiver of an unknown type, high resolution Fast Fourier Transform (FFT) blocks implemented via large field programmable gate arrays (FPGAs) are used so as to accurately detect the intended signal, and the jamming signal is adjusted accordingly. The use of large FPGAs to implement high resolution FFT blocks may cause a variety of issues, however. For example, such FPGAs may consume a large amount of power, and may be expensive.
Consequently, development of new jammers that reduce the chance of jamming wanted communications, that increase the chance of jamming desired intended signals, and that may allow the use of cheaper FFT blocks are desirable.